Since its inception more than fifty years ago, silicon (Si) photovoltaic (PV) technology has been a reliable source of power for military and commercial satellites in space (See, e.g., P. IIes in Progress in Photovoltaics: Research and Applications, Vol. 2, 95 (1994)). However, more recently, the III-V compound semiconductor technology has developed with the availability of high-throughput thin film deposition systems on germanium substrates and subsequent fabrication of high-efficiency (˜25%), multi-junction solar cells (See, e.g., P. R. Sharps, et al., IEEE PVSC 23, 650 (1993)).
Silicon (Si) solar cells for use in space environments have not experienced comparable improvements and, as a result, have lost a significant market share to the compound semiconductor multi-junction solar cell technology. However, radiation-tolerant Si solar cells with efficiencies (˜20-25%) remain a viable option for several space applications including nanosatellites, unmanned space vehicles, and commercial satellites not requiring high efficiencies (See, e.g., P. IIes in Progress in Photovoltaics: Research and Applications, Vol. 8, 2864 (2000)). Additionally, thin-film Si solar cells also offer significant cost savings in manufacturing and launch expenses (See, e.g., J. Tringe et al., IEEE PVSC 28, 1242 (2000)). The performance of Si solar cells in space environments is severely degraded due to radiation of high-energy particles and electromagnetic radiation (See, e.g., M. Y. Yamaguchi, et al., Appl. Phys. Lett. 68, 3141 (1996)). The radiation-induced surface and volume damage creates volume recombination centers and reduces minority carrier diffusion lengths, resulting in a significant reduction of the cell performance in the near infrared (IR) region (See, e.g., L. Prat et al., Solar Cells 31, 47 (1991)). Improvements in Si space solar cells have been achieved by incorporation of back surface fields, surface texturing, fine grid spacing, and thinner substrates (See e.g., A. Suzuki et al., IEEE Trans. Elect. Dev. 46, 2126 (1999)). It has been well established that tolerance to ionizing radiation-induced recombination losses of Si solar cells is significantly improved by reducing cell thickness (See, e.g., S. Matsuda, et al., ESA, SP 320, 609 (1991)). Decreasing Si thickness also lowers weight.
Ideally, the cell optimum thickness is a fraction of the minority carrier diffusion length (See, e.g., H. J. Hovel, Solar Cells, Semiconductors and Semimetals, Vol. II. Academic press (1975)). However, in the near IR (˜0.9-1.11-μm) wavelength range, optical absorption in Si is weak (See, e.g., M. A. Green and M. J. Keevers, Progress in Photovoltaics: Research and Applications, Vol. 3, 189 (1995)). Random, wet-chemical texturing techniques have been used to form pyramids in the (100) Si crystal orientation for reducing reflection and enhancing optical absorption by increased oblique coupling into the solar cells (See, e.g., P. Campbell and M. A. Green, Appl. Phys. Lett. 62, 243 (1987)). Applicability of these texturing schemes to thin wafer and films (˜20-50 μm) is limited due to their large dimensions and preferential (100) crystal etching mechanisms.
Alternate approaches based on subwavelength random, or periodic microscopic structures aimed at reflection reduction and enhanced near-IR absorption have been extensively investigated. Randomly textured subwavelength surfaces reduce broadband reflection to <5% and enhance near-IR absorption through increased oblique coupling of light into the semiconductor (See, e.g., Saleem H. Zaidi et al., IEEE Trans. Elect. Dev. 48, 1200 (2001)). Random subwavelength surfaces represent a Fourier summation of a wide range of periodic microscopic structures, the enhanced near-IR absorption from such surfaces results from diffractive coupling of light as opposed to refractive oblique coupling in geometrically textured surfaces.
In contrast with random subwavelength microscopic structures, periodic subwavelength microscopic structures offer highly controllable mechanisms aimed at Si reflection and absorption response over a wide spectral range. T. K. Gaylord et al. in Appl. Opt. 25, 4562 (1986) have described rigorous models of rectangular profiled grating microscopic structures exhibiting zero reflection for a suitable choice of grating parameters. D. H. Raguin and G. M. Morris in Appl. Opt. 32, 1154 (1993) have determined broadband anti-reflection properties of 1D triangular and 2D pyramidal surfaces. Ping Shen et al. in Appl. Phys. Lett. 43, 579 (1983) have reported wavelength-selective absorption enhancement of thin-film (˜2 μm) amorphous Si solar cells by grating coupling into waveguide modes. C. Heine and R. H. Morf in Appl. Opt. 34, 2478 (1983) have demonstrated enhanced near IR absorption in ˜20-μm thick Si films by diffractive coupling. Broadband and narrowband spectral reflection response of subwavelength Si grating microscopic structures has been reported by Saleem H. Zaidi et al. in J. Appl. Phys. 80. 6997 (1996). Enhanced near IR response of subwavelength grating solar cells has also recently been demonstrated by Saleem H. Zaidi et al., in IEEE PVSC 28, 395 (2000).
Gaylord et al., supra, describes the anti-reflection properties of 1D rectangular grating microscopic structures; however, the need to create absorption close to the solar cell junction particularly in near IR spectral range is not discussed. Heine and Morf, supra, describe a diffractive approach directed at improving solar cell response at λ˜1.0 μm. For thin-film solar cells, near IR absorption is weak due to the indirect bandgap of Si. By fabricating a grating structure at the back surface of the cell, enhanced absorption can be achieved by efficient coupling of the incident beam into two diffraction orders for a symmetric profile, or a single diffraction order for a blazed profile. Heine and Morf teach away from the use of a front surface grating because of surface passivation issues. By proper design of grating parameters, Heine and Morf have chosen the direction of propagation of diffraction orders such that at angles larger than the critical angle, these orders are trapped as a result of total internal reflection.
The concept of improving electron-hole pair (EHP) collection in the volume of a solar cell using deeply etched trenches was investigated in (110) Si solar cells for the purpose of improving radiation tolerance (See, e.g., John Wohlgemuth and A. Scheinine, IEEE Photovoltaic Specialists Conference, page 151 (1980)). Because of the preferential etch differential between <111> and <110> planes, simple wet-chemical etching chemistry can be employed to form one-dimensional trenches in (110) Si (See, e.g., Saleem H. Zaidi et al., J. Appl. Phys. 80, 6997 (1996). In IEEE PVSC 28, 1293 (2000) trenches formed in (100) Si using deep reactive ion etching techniques were investigated by H. Presting et al. The structures employed were macroscopic (>> optical wavelengths) and the observed improvements were marginal, presumably, the result of a lack of enhanced near-IR absorption. In both (110) Si vertical grooves, and (100) Si deep random ion etching (DRIE) trenches, a significant fraction of the EHPs generated in the volume of the cell is lost to recombination due to the inability of the material to absorb near IR radiation near the junction areas.
Accordingly, it is an object of the present invention to improve light absorption in thin films (<50 μm) used as solar cells and photodetectors in the near-IR spectral range.
Another object of the present invention is to enhance volume collection of electron/hole pairs in solar cells and photodetectors used in the presence of ionizing radiation.
Additional objects, advantages and novel features of the invention will be set forth, in part, in the description that follows, and, in part, will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.